Combustion gases produced in the combustion chamber of an internal combustion engine are commonly forced under pressure to flow past the piston of the engine and into the engine's crankcase. Such gas is called "blow-by" gas, and usually consists at least partially of an air/oil mist. The reciprocation of the piston typically causes pressure fluctuations in the crankcase, causing the air/oil mist in the crankcase to be routed through an air cleaner having an air filter and back to the intake of the carburetor. It is undesirable to have oil in the combustion chamber because this leads to accumulations of oil in the valves and combustion chamber which may interfere with the operation of the engine. Also, the air filter in the air cleaner must be replaced frequently due to larger amounts of oil suspended in the air passing through the air cleaner, adding maintenance expense.
To alleviate this problem, it is customary in a four cycle engine to provide a breather system for separating the oil from the air/oil mist prior to recycling the air through the air cleaner and the carburetor. In a typical breather system as shown in U.S. Pat. No. 4,169,432, the oil mist in the crankcase is subjected to positive pressure when the piston travels in a downstroke, and the oil mist is forced through a breather passage containing a check or breather valve to an oil separation chamber where the oil mist is separated from the gas. On the upstroke of the piston, the pressure in the crankcase changes from positive to negative and the separated oil is drawn back into the crankcase through a return passage.